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Three-Lined Pencilfish: A Comprehensive Care Guide (Nannostomus trifasciatus)

The Three-Lined Pencilfish (Nannostomus trifasciatus) is an elegant and distinctive freshwater fish known for its slender body and bold horizontal striping. Native to South America, this species is admired for its graceful swimming behavior and subtle yet attractive coloration.

Three-Lined Pencilfish display three dark horizontal stripes running along their bodies, which give the species its common name. Like other pencilfish, they often swim at a slight angle near the surface and move gently through the aquarium rather than forming tight schools.

Because of their peaceful nature and unique appearance, Three-Lined Pencilfish are a popular choice for planted aquariums and South American community tanks.

Natural Habitat and Distribution

Three-Lined Pencilfish originate from South America, where they inhabit parts of the Amazon and Orinoco river basins across countries such as Brazil, Peru, Colombia, and Venezuela.

In the wild they are commonly found in:

  • Slow-moving forest streams

  • Blackwater tributaries

  • Flooded forest habitats

  • Calm waters with dense vegetation and leaf litter

These environments often contain soft, acidic water and subdued lighting created by overhanging forest canopy.

Size

Adult size: approximately 1.5–2 inches (3.5–5 cm)

Three-Lined Pencilfish grow slightly larger than some other pencilfish species but still remain relatively small.

Tank Size

Minimum tank size: 15 gallons
Recommended tank size: 20 gallons or larger

Three-Lined Pencilfish should be kept in groups of at least 6–8 fish, though larger groups often produce more natural behavior.

Providing a longer aquarium allows them to display their graceful swimming patterns.

Water Parameters

Three-Lined Pencilfish prefer stable water conditions similar to those found in their natural habitat.

Temperature: 72–80°F
pH: 5.0–7.0
Hardness: soft to moderately soft

Maintaining clean water and performing regular water changes helps keep them healthy.

Aquarium Setup

A planted aquarium with natural decorations works very well for Three-Lined Pencilfish.

Recommended tank features include:

  • Dense aquatic plants

  • Floating plants to diffuse lighting

  • Driftwood and natural decorations

  • Dark substrate

Providing both plant cover and open swimming areas allows them to feel secure while displaying natural behaviors.

Diet

Three-Lined Pencilfish are omnivorous micro-predators that feed on small foods.

In the aquarium they will accept:

  • High-quality micro pellets

  • Crushed flakes

  • Frozen foods such as baby brine shrimp or daphnia

  • Small live foods

Because of their small mouths, food should be finely sized.

Behavior and Temperament

Three-Lined Pencilfish are peaceful and calm fish that typically inhabit the upper and middle levels of the aquarium.

Typical behaviors include:

  • Slow, graceful swimming near the surface

  • Loose group formation rather than tight schooling

  • Gentle interaction with tank mates

They are non-aggressive and generally easy to keep in community aquariums.

Tank Mates

Because of their peaceful temperament, Three-Lined Pencilfish work well with other small community fish.

Good tank mates include:

  • Small tetras

  • Rasboras

  • Corydoras

  • Dwarf shrimp

  • Other peaceful nano fish

Avoid aggressive species or large fish that may view them as prey.

Breeding

Three-Lined Pencilfish are egg-scattering fish that may spawn in well-planted aquariums.

During spawning:

  • Eggs are scattered among plants or moss

  • Adults do not guard the eggs

  • Eggs usually hatch within 24–48 hours

The fry are extremely small and require infusoria or powdered fry food before transitioning to baby brine shrimp.

Fun Facts

  • The name trifasciatus refers to the three dark stripes along the body.

  • Pencilfish often swim at a slight upward angle near the surface.

  • Their calm swimming style gives them a very different appearance compared to fast-moving schooling fish like many tetras.

Conclusion

The Three-Lined Pencilfish (Nannostomus trifasciatus) is a graceful and peaceful fish that works beautifully in planted aquariums and South American community setups. Their distinctive striping, calm temperament, and elegant swimming style make them a unique addition to freshwater aquariums. With proper care and stable water conditions, Three-Lined Pencilfish can thrive and add subtle beauty to a well-designed tank.

Looking to Add Fish to Your Aquarium?

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Pencilfish (Genus Nannostomus): A Comprehensive Guide

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